GRESHAM JCT's Sequence Switches Explained

 Years ago I wrote up a profile of METRA's GRESHAM JUNCTION tower on the Rock Island division south of Chicago.  Not only had GRESHAM JUNCTION managed to stay open into the 21st Century, it was a unique North American example of a sequence switch interlocking. Supplied by the Standard Telephone and Cables Company of London, the interlocking used telecom grade sequence switches to carry out the interlocking functions instead of relays.

In my original post I provide photos of the equipment as well as a general concept of how sequence switches work and some sequence switch interlocking circuit diagrams from the UK, but without time and access to the equipment or someone who was intimately knowledgeable about how it functions, my commentary had to remain at a very high level.  Fortunately, the gang from the Connections Museum in Seattle is on the case because sequence switches is pretty much how Bell Number 1 Panel central office machines functioned (again, as opposed to later relay based technologies). I could try to go into things, but fortunately the museum's YouTube channel has video that is specifically about how sequence switches work. 

Once you see them in action at the Connections Museum, their function in the photos from GRESHAM JCT will become obvious. Of particular note is the function of the magnetic clutch mechanism that rotates the switch spindle. Perhaps if I stare at things enough I can determine what each sequence switch corresponds to in the interlocking.  Is each a complete route? An entrance? An exit? An entrance-exit combination? Let me know in the comments if you figure it out.  BTW, if you like this video take some time to watch the rest of the Museum's content.  It's top notch and does a superb job of explaining some normally opaque topics in the realm of pre-modern telephone switching technology.

Call to the Signal Bungalow

So a number a years ago I was planning to get some signal photos along the former Conrail Buffalo Line and while searching for information on CP-NORTH MILLER, I noticed something strange about the results.

It appears that, for some bizarre reason, the telephone in the signal house at CP-NORTH MILLER had gotten itself listed in a phone book at some point and now it was plastered all over various cyber-leech clickbait websites.  Normally I might not have given this a second thought, but for another poignant experience I had way back in 2006.

On another Buffalo Line trip, while taking photos at CP-LINDEN out of the blue an old style bell phone began to ring inside the spacious 1950's vintage PRR CTC-style relay house.  I had a chuckle thinking who would be calling an interlocking in the middle of nowhere, but when I saw the phone listing for CP-NORTH MILLER 8 years later, I just HAD to try it.

The PRR was never very enthusiastic about CTC, content to rest on its laurels of multi-track main lines, manned block stations and the manual block system.  However it did green light a few projects and the Main Line between Rockville to Buffalo, was one such example.  Installed in 1957, the Buffalo Line CTC was definitely a creature of the PRR with lavish signal huts, a reliable power supply (so no approach lighting) and apparently, a PTSN station in each walk-in signal shanty.

So back in 2014 I drove up from Rockville to Millersburg, all excited about capturing a cool intersection of the rail and telephone network communities of interest and...nothing happened.  I tried the number and the phone didn't ring.  Ah well, should have known it wouldn't have worked.

Fast forward to 2017 and I was back up again, chasing signals on the Buffalo Line and I just couldn't help myself to pull over and see if I could give the call another go.  At this point I'll cut to the video.



Man, what ever happened to that wonderful rich 1950's ringer sound?  Absolutely wonderful!  I'm going to go out on a limb and say I'd bet it was a wall mounted Western Electric model 554.  I don't see the PRR sticking a desk set in such a cramped location, nor do I see Conrail or NS ever having upgraded the line for DTMF service ;-)  Of course a Western Electric rotary phone is probably even more reliable than the US&S glass case relays powering the interlocking logic. 

Now For Something Completely The Same (#1AESS Retirement)

I just got word that on or about right now, the last active AT&T/Western Electric #1AESS telephone switches (generally confined to the former SBC territory) are being taken out of service.  This matters because the railroads are currently speeding towards the completely sterile, 100% digital environment that the telecoms have apparently just achieved.  Unlike railroads, which still have a few instances of electro-mechanical interlocking machines and hundreds of relay plants in service, the teleco's banished the last electro-mechanical switch from their network back in 2002, with the majority of the work taking place between 1970 and the 1990.  Back in the day, there was an entire scene of people who would go from place to place, listening to all the strange ways that the phone system functioned.  Today this sort of task would be a fools errand because everything is the same across the entirety of North America. 

Until recently, the #1ESS and #1AESS were the only exceptions.  Sort of like the 3400 series L cars or the NYCTA R68's and their cam-controlled DC propulsion systems, they were analog machines build in a digital world.  While the higher order functions were computer controlled, the actual switching was carried out using reed relays.  It's basically the N-X CTC panel of telephone switches with a computer bolted on to handle some of the route selection functions.  Even then, the attached were old school enough to fall into the "cool" category. 

Like with classic railroad signaling surviving on out of the way branch lines, the #1A aand #1 switches held on due to the recent wholesale divestment in copper wire / landline telephone services.  Still, the economics eventually because just too lopsided in favor of replacement and AT&T canceled its support contract with Western Electric successor Lucent in 2015, with an expected retirement date of 2017.  

This is why it's so important to get out and capture the classic technology before its just collecting dust in the corner of a museum.  In a few decades just about everything will be software running on some bog standard processor mounted on a raspberry pi.